A microporous film having excellent chemical stability and superior physical properties is broadly used as a battery separator, a separation filter, a micro-filtration film, or the like.
A method for preparing a microporous film based on polyolefin may be generally classified into four (4) groups: first, formation of a microporous film in a non-woven fabric form by preparing a thin polyolefin fiber; second, a dry process that includes preparing a polyolefin film, stretching the film at a low temperature and causing micro-cracks between lamella, which are a crystal part of the polyolefin, to form micropores; third, a process of forming pores at an interface between polyolefin and a filler during stretching by introducing the filler, for example, an inorganic material, an organic material non-compatible with polyolefin, or the like, into a polyolefin resin. Three of the methods have favorable features such as formation of pores, which proposes desired permeability and reduced production costs to achieve economical benefits, but also have disadvantages such as low mechanical strength, irregular and large pore size, and difficulties in securing quality uniformity. Alternatively, a fourth method is a wet process that includes mixing a polyolefin resin with a diluent (a low molecular weight organic material having a molecular structure similar to polyolefin) at a high temperature at which the polyolefin resin melts as a single phase, occurring phase separation into the polyolefin and the diluent during cooling, and then, extracting the diluent part, to thereby form pores in a resultant film. The wet process generally provides desired mechanical strength and permeability through stretching/extraction after phase separation, has advantageous features such as providing a film having a small and uniform thickness, compared to previously described methods, formation of pores having a uniform size, securing excellent physical properties, or the like, therefore, is widely used in fabricating a separator for a secondary battery such as a lithium ion battery.
With continuous development of high performance secondary batteries since they began to be used in earnest, a battery separator fabricated by a wet process is also increasingly used. In addition, efforts to improve productivity of a microporous film and characteristics of a film by the wet process have been continuously conducted. A representative method may include using ultra-high molecular weight polyolefin (UHMWPO) having a weight-average molecular weight of about 1,000,000 or adding the same to a raw material to increase a molecular weight of a composition to be prepared and using a stretching process to increase a strength of a porous film.
Regarding this, U.S. Pat. Nos. 5,051,183, 5,830,554, 6,245,272 and 6,566,012 describe a method of fabricating a microporous film by using a composition that contains polyolefin having a weight-average molecular weight of not less than 500,000 and a diluent dissolving the polyolefin at a high temperature to prepare a sheet, and then, sequentially conducting a stretching process and a diluent extraction process. In these patents, the stretching process is conducted by uniaxial or biaxial stretching and using a typical tenter, roll, calendar, or the like, or a combination thereof. In relation to the biaxial stretching, the foregoing patents generally describe that both of simultaneous biaxial stretching and sequential biaxial stretching may be used. However, overall examples recited in the above patents are particularly restricted to the simultaneous biaxial stretching or simply describe a biaxial stretching, however, have not specifically defined about stretching temperature in each of the machine and transverse directions. In other words, features of a sequentially biaxial stretching process by using a roll to stretch to the machine direction, and then, to stretch to a transverse direction by using a tenter, features of a simultaneously biaxial stretching process, and differences therebetween were not disclosed in the foregoing patents.
In recent years, among microporous polyolefin films commercially available in the market, products manufactured by a wet process, which are recognized to have excellent performance, may include one stretched after extracting the diluent and the other stretched before extracting diluent. The former has pores formed while extracting the diluent, which are prone to be deformed during stretching, therefore, the stretching process could hardly improve physical properties. Further, since flexibility of the polyolefin free from the diluent is not proper, the stretching process is not easily conducted. On the other hand, the latter allows the polyolefin to become flexible by the diluents, thus allowing the stretching process to be much more easily conducted and securing excellent production stability. Moreover, since a thickness of a film is reduced by stretching, the diluent may be easily removed from the film while extracting after stretching. However, most of the commercialized products formed via the above stretching process before extracting are currently well known in the art as products manufactured by a simultaneously biaxial stretching process.
A simultaneously biaxial stretching process is a stretching method that uses a chuck (an biting device) to grip and fix the top and bottom sides of a sheet prepared by mixing polyolefin and a diluent, similar to thumbs and index fingers of hands of a human, and broadens the gap between chucks in both a machine direction and a transverse direction, simultaneously. In case of the simultaneously biaxial stretching process, a gap between chucks is narrow before stretching, however, broadened by a specific draw ratio after stretching, thus increasing an area not gripped by the chucks (see FIG. 1). Such a non-gripped area has a lower actual stretching ratio than that of a gripped area, to cause non-uniformity in qualities between the gripped area and the non-gripped area. The above tendency is more serious in the case where a stretching ratio is increased, a stretching temperature is decreased for enhancing a mechanical strength, and a sheet before stretching has a relatively large thickness, or the like. For this reason, when a thick film having a high strength is fabricated by the simultaneously biaxial stretching process, a problem of deteriorated uniformity in quality may be encountered. As the application of lithium ion secondary batteries is increased from small electronics to laptop computers, electronic tools, hybrid cars, or the like, a battery separator having high strength and a high thickness is required. Therefore, when a microporous film is fabricated, a stretching ratio and/or a thickness of a sheet before stretching must be increased. However, the simultaneous biaxial drawing process has disadvantages in that quality uniformity is reduced when the sheet is thick and rigid, which can be easily disengaged from the grip chuck, thus causing a decrease in productivity stability.
Further, as for the simultaneously biaxial stretching process, due to difficulties in designing a machine and limitation in equipment costs, it is difficult to design the machine having a variable stretching ratio and apply this in commercial production lines. That is, it is not easy to manufacture commercial products having different characteristics by controlling the stretching ratio.
Korean Patent No. 10-0599898, Japanese Patent No. 2002-088188, Japanese Patent No. 2010-00705, and Japanese Patent No. 2009-226736, respectively, disclose a sequentially biaxial stretching process to sequentially conduct stretching to the machine direction and to the transverse direction. A method of fabricating a battery separator through the sequentially biaxial stretching process disclosed in Korean Patent No. 10-0599898, Japanese Patent No. 2002-088188 and Japanese Patent No. 2010-00705 includes melting/mixing polyolefin, a diluent, and inorganic particles to prepare a sheet, extracting the diluent and the inorganic particles with a solvent to form pores and sequentially stretching the sheet to the machine and transverse directions to enhance a mechanical strength and permeability of the film. However, this method has a limitation in improving the mechanical strength via stretching. Accordingly, the separator with high strength cannot be fabricated via this method. Compared to a mechanical strength improved by orientation of a polyolefin resin during stretching, the deformation of pores easily occurs to enlarge pores. That is, since a size of the pore and permeability of the film tend to excessively increase whereas the mechanical strength is slightly enhanced, an increase in a stretching ratio above a critical level is restricted. In other words, it is difficult to manufacture products having a wide range of mechanical strengths (in particular, a product having puncture strength of not less than 0.25N/μm) and, when a pore size is excessively increased, quality uniformity is seldom secured. The method disclosed in Japanese Patent No. 2009-226736 includes preparing a polyolefin sheet, forming micro-cracks inside crystals by a dry process during stretching to the transverse direction and enlarging the micro-cracks during stretching to the transverse direction, thus providing desired permeability. Compared to formation of pores by using inorganic particles as above described in the foregoing patents, a size of the pore is reduced and a stretching ratio may be increased. However, it is difficult to overcome limitations of a battery separator fabricated by the dry process such as low mechanical strength and reduced quality uniformity.
In view of such circumstances, the present invention proposes application of a sequentially biaxial stretching process to fabricate microporous polyolefin films so that quality uniformity and production stability during manufacturing may be secured, a variety of properties may be provided by controlling physical properties, permeability, shrinking property, or the like, and, in particular, provides a method of fabricating a high strength battery separator.